A resolver/digital converter (RD converter) is generally used in a way of detecting rotation of a rotation detector connected to an object to be monitored as rotation detection signals, of converting the rotation detection signals into angle values and of outputting angular position of the object to be monitored by digitalized angle values.
FIG. 8 shows a prior art angle/digital converting apparatus 100. The angle/digital converting apparatus 100 is basically composed of a rotation detector 10, a tracking type R/D converter 20 and an exciting signal generator 30.
The exciting signal generator 30 outputs an exciting signal f(t)=sin ωt, a periodic function where t represents time and ω represents an angular velocity, to excite the rotation detector 10. Beside that, it shapes a waveform of the exciting signal f(t)=sin ωt and supplies it to a synchronous detector 25 provided in the tracking type R/D converter 20 as a reference signal f(t)′. It is noted that a waveform shaper 29 shapes the waveform of the exciting signal f(t) into a rectangular reference signal f(t)′. Accordingly, the reference f(t)′ has the same period 2π/ω as that of the exciting signal f(t).
The rotation detector 10 outputs two quadrature rotation detection signals S1=f(t)sin θ and S2=f(t)cos θ and inputs the quadrature rotation detection signals S1 and S2 into the tracking type R/D converter 20.
The tracking type R/D converter 20 arithmetically operates the quadrature rotation detection signals S1 and S2 to output an output angle signal φ. The output angle signal φ is negatively fed back to the input side so as to control that the relationship between the output angle signal φ and the input angle signal θ of the tracking type R/D converter 20 is kept to be θ=φ.
That is, the tracking type R/D converter 20 multiplies the quadrature rotation detection signals S1=f(t)sin θ and S2=f(t)cos θ with the feedback signals cos φ and sin φ from feedback loops 28A and 28B by multipliers 22A and 22B to obtain f(t)sin θ cos φ and f(t)cos θ sin φ. Then, a subtractor 23 subtracts them from each other to obtain a subtracted value f(t)sin(θ−φ). A synchronous detector 25 synchronously detects this subtracted value to produce a signal sin(θ−φ) as a control deviation ε=sin(θ−φ) in which the exciting signal component f(t) has been removed. The tracking type R/D converter 20 inputs this control deviation ε to a voltage controlled oscillator 26 to convert into a pulse train having a frequency corresponding to the value of the control deviation ε. A counter 27 counts this pulse train and outputs its counted value as an output angle φ. The RD converter operated with this angle/digital conversion method is called as a tracking type R/D converter in general as disclosed in Japanese Patent Application Laid-Open No. 2000-353957 for example.
The voltage controlled oscillator 26 outputs positive pulses when the synchronously detected output of the synchronous detector 25 is positive and outputs negative pulses when the synchronously detected output is negative in this tracking type R/D converter. The counter 27 is composed of an up/down counter and counts up while the positive pulses are inputted and counts down while negative pulses are inputted.
The control deviation ε becomes 0 in a state in which the input angle θ and the output angle φ outputted from the counter 27 hold θ=φ and an equilibrium state is kept in this state. When the input angle θ changes to θ′ in the equilibrium state, the control deviation ε becomes ε≠0 and the voltage controlled oscillator 26 outputs positive or negative pulses in accordance to the polarity of the control deviation ε. The counter 27 counts that pulses and the control deviation ε becomes ε=0, i.e., the equilibrium state, at a point of time when the output angle φ reaches to a relationship of φ=θ′. Thus, the tracking type R/D converter 20 outputs an output angle φ that varies following the input angle θ.
The basis of the tracking loop method that converts the analog rotation detection signals into the digital angle is that it uses sin(θ−φ) as the control deviation ε as described above and this is not exceptional also in case of Japanese Patent Application Laid-Open No. 2000-353957. Although it is possible to zero the target deviation between the input angle θ and the output angle φ by controlling so as to render sin(θ−φ) to zero, a zero point also exists at θ−φ=180° besides the zero point at θ−φ=0 according to the characteristics of the sin function. The tracking loop becomes stable in the state of θ−φ=180° because the control deviation ε is zero even though an angle error between the input angle θ and the output angle φ is maximum. Therefore, there has been a problem that when large changes of the input angle occur in a short time, the control deviation ε becomes small and a control response drops when the angle error (θ−φ) approaches the vicinity of 180° even though the angle error is large. In a worst case, a serious drawback is brought about when the object to be monitored stops in a state when the rotation angle has changed by 180° at speed exceeding the control response such that the object is left in a hang-up state having this angle deviation.